When operating underwater oil wells, that it is desirable to fit each well with a pumping station. The station must be an underwater pumping station. It is desirable to supply power to the pumping station from a remote location, possibly from the shore. That requires powering the pumping station by means of a power cable, running from a power supply generator. Since the motor of the pump is an induction motor, it is powered directly by a power supply generator delivering three-phase AC thereto at variable frequency via a cable system essentially comprising a three-conductor power cable. By way of numerical example, the motor and pump unit may consume up to 2500 kW, i.e. 3500 kVA, or about 300 A at 6.6 kV at a frequency of 100 Hz. Nevertheless, applications are already being envisioned at 6000 kW.
A voltage-raising transformer and a voltage-lowering transformer may be inserted at respective ends of the cable to reduce losses.
In the pumping station, adjacent to the motor and pump unit, various items of auxiliary equipment are provided including electronics cards that operate on DC and at low power, and valves, fans, or pumps, operating on AC at medium power.
This means that such auxiliary equipment needs to be supplied power and that control and information signals need to be interchanged therewith.
The conventional solution is either to add a cable for this purpose adjacent to the power cable, or to incorporate additional conductors in the power cable.
Documents FR-A-2 699 016, FR-A-2 506 969, and FR-A-2 230 141 also disclose, in various technical fields, that it is possible to avoid using additional electrical conductors for powering auxiliary equipment associated with a load that is powered with AC. The solution suggested in those documents is to supply power to the auxiliary equipment by means of a current transformer having its primary winding in series with the load. Document FR-A-2 699 016 suggests in particular providing such a current transformer for each phase. All three documents mentioned indicate that the current in the secondary winding must be rectified and regulated in order to obtain satisfactory DC power supply for the auxiliary equipment. In particular, documents FR-A-2 699 016 and FR-A-2 230 141 suggests providing in cascade a rectifier and a voltage limiter that diverts the excess current supplied by the secondary winding.
Nevertheless, it is not practical to apply this technique to the case of a remotely-powered pumping station because of the above-mentioned electrical characteristics of pumping stations. The amount of excess current can be very great, and conventional limiting means, as indeed the techniques of documents FR-A-2 699 016 and FR-A-2 230 141 lead to excessive amounts of power being dissipated.